1. Field of the Invention
This invention relates to illumination systems for optical readers such as bar code scanners. More particularly, this invention relates to an improved light transmitting and receiving apparatus for use in optical readers.
2. Description of the Prior Art
Modern optical reading devices such as bar code scanners are required to read relatively wide bar code symbols while remaining in a stationary position at the instant of reading. It is also desirable that the system optics provide a large depth of field in order to read curved surfaces, or surfaces otherwise having uncertain displacement from the reading device, and to generally provide increased ease of reading. At the same time there is a need to conserve power, a goal which is limited by the requirements of the optics, the sensitivity of the photodetector, and the minimal acceptable signal-to-noise ratio.
In U.S. Pat. No. 4,766,300 (Chadima), there is disclosed a portable bar code reader which employs an elliptical reflector that is elongated in a width dimension of the bar code reader so as to illuminate a region located in front of an optical window and containing a bar code symbol. In this device, light rays diverge from a flash tube illumination source toward a surface to be scanned, and are reflected in a receive path that passes above and beyond the reflector in a generally rearward direction. This arrangement results in a relatively large angle between the illumination path and the receive path. Also, across the scan width there is significant off-axis falloff of illumination intensity, which limits the ability to read indicia that are situated at the ends of a wide sensing region.
In U.S. Pat. No. 4,891,739 (Yasuda), an optical arrangement is shown for uniformly illuminating a planar surface using a curved reflecting mirror by a non-Lambertian arcuate or point light source. A light shielding member is required to prevent light from the source from directly reaching the object to be illuminated.
In U.S. Pat. No. 5,233,171 (Baldwin) there is disclosed an optical code reader of the stationary type in which LED light sources are connected together and used as paired elements.
In French Patent Publication No. 2,700,404 (Massieu), there is disclosed a bar code illumination system which illuminates a bar code symbol via a plurality of lens segments and receives image data through an opening between the lens segments.
In Japanese patent abstract 63-56768 (Nishijima), there is shown a bar code scanner which illuminates a bar code symbol via a reflector that includes a centrally disposed opening through which-image data is received.